1. Field of the Invention
The invention relates to a polymer composition charged with a powdered mineral material which is in an insoluble granular form but which is inflatable in water, composed of at least one crosslinked polymer prepared from at least one unsaturated ethylene monomer having at least one carboxylic, amide, sulfuric, sulfonic, phosphoric, phosphonic or hydroxyl functional group and at least one inert, powdered mineral charge which is well dispersed within and intimately bonded to the polymer material, and to a method for obtaining said charged granular composition.
b 2. Discussion of the Background
For some time, the specialized literature has published numerous formulations of particulate polymer materials containing inert mineral charges as well as the methods enabling the production of these compositions.
These particulate polymer materials, which are in the form of gelatinous granules or which have a certain elasticity, have, in a well-known manner, the property of absorbing several times their weight of water when they are placed in contact with an aqueous medium through immersion or aspersion for example, and of slowly releasing the absorbed water when the saturated granules are incorporated in a plant growth medium. In this manner, the absorption of a large quantity of water by these particulate polymer materials (incorporated in a plant growth medium) during rainfall or artificial watering, enables slow release of the accumulated water into the surrounding medium and, consequently, the creation of an almost permanent damp medium which is suitable for plant development.
In addition, through this mechanical expansion-contraction effect, the water absorption and desorption cycles of the particulate polymer materials, which cause variations in their apparent volume, promote the aeration of the plant growth media in which they are incorporated because of voids created by the expansion-contraction cycle.
Finally, the particulate polymer materials having a high water absorption and desorption capability have also been described as useful in agricultural preparations which include fertilizers and/or biocidal agents.
Such particulate polymer materials combined with inert mineral charges are described in French Pat. No. 2,173,934 and are obtained by a method which consists of combining a mixture of at least one powdered hydrophilic and water-soluble polymer and an inert powdered charge with a view to covering the contact surface of said polymer, then of submitting said mixture, in the presence of a certain quantity of water, to the action of ionizing radiation in order to cross-link the polymer.
In spite of all the advantages of the products produced by this method, the particulate polymer materials associated with inert mineral charges always demonstrate certain disadvantages which can be considered as being major.
A first major disadvantage resides, for example, in the fact that these particulate polymer materials are formed from a quantity of at least one particulate polymer, which comprises the core or nucleus of each particle of the product and of a layer of powdered inert mineral material which is coated on and adheres to the particulate polymer material base. The mineral layer forms a protective screen against the natural photo-destruction of ultraviolet radiation.
However, the layer of powdered mineral material which adheres to the particles of the particulate polymer material can be naturally discontinuous, leaving open areas of polymer material not covered by mineral material which areas of polymer material are exposed to the action of destructive photons. In addition, the cycles of expansion and contraction which the particulate polymer materials experience, can cause disintegration of the inert mineral matter adhering more or less strongly to the polymer material, and these cycles break the protective mineral layers of the particulate polymer material, rendering it particularly sensitive to photodegradation by ultraviolet radiation.
Further, and as the phenomenon has been observed, during the water absorption and desorption cycles, the particulate polymer materials lose part of their capability to take in water and this capability decreases more rapidly because the particulate polymer material, losing its protective screen, is sensitive to photodegradation, thus causing premature disappearance through destruction of the particulate polymer materials used in plant growth media.
Other particulate polymer materials associated with inert mineral charges are also described by their compositions and by the methods of preparing the same in Patent EP 0,072,213. Such particulate compositions which are useful as an additive for plant growth media comprise gel particles which consist of an acrylamide copolymer and a (meth)acrylic acid salt, in which the molar ratio is at least 70:30 but not greater than 95:5, and are chemically cross-linked so that the amount of water-soluble polymer is less than 30% by weight of the dry polymer, with the external surface of said gel particles being modified by means of a finely particulate hydrophilic agent such as kaolin, fuller's earth, talc, bentonite and aluminum silicate. However, these chemically cross-linked particulate polymer materials having mineral coating charges exhibit the same major disadvantages as those noted in the above-discussed reference. A need therefore continues to exist for a mineral containing polymer product of improved stability properties.